R. Puzniak

Carbon substitution in MgB2 single crystals: Structural and superconducting properties

The growth of carbon-substituted magnesium diboride MgsB1˛xCxd2 single crystals with 0 x 0.15 is reported, and the structural, transport, and magnetization data are presented. The superconducting transition temperature decreases monotonically with increasing carbon content in the full investigated range of substitution. By adjusting the nominal composition, Tc of substituted crystals can be tuned in a wide temperature range between 10 and 39 K. Simultaneous introduction of disorder by carbon substitution and significant increase of the upper critical field Hc2 is observed. Comparing with the nonsubstituted compound, Hc2 at 15 K for x = 0.05 is enhanced by more than a factor of 2 for H oriented both perpendicular and parallel to the ab plane. This enhancement is accompanied by a reduction of the Hc2-anisotropy coefficient g from 4.5 sfor the nonsubstituted compoundd to 3.4 and 2.8 for the crystals with x = 0.05 and 0.095, respectively. At temperatures below 10 K, the single crystal with larger carbon content shows Hc2 sdefined at zero resistanced higher than 7 and 24 T for H oriented perpendicular and parallel to the ab plane, respectively. Observed increase of Hc2 cannot be explained by the change in the coherence length due to the disorder-induced decrease of the mean free path only.

High magnetic-field scales and critical currents in SmFeAs(O, F) crystals

With the discovery of new superconducting materials, such as the iron pnictides, exploring their potential for applications is one of the foremost tasks. Even if the critical temperature T(c) is high, intrinsic electronic properties might render applications difficult, particularly if extreme electronic anisotropy prevents effective pinning of vortices and thus severely limits the critical current density, a problem well known for cuprates. Although many questions concerning microscopic electronic properties of the iron pnictides have been successfully addressed and estimates point to a very high upper critical field, their application potential is less clear. Thus, we focus here on the critical currents, their anisotropy and the onset of electrical dissipation in high magnetic fields up to 65 T. Our detailed study of the transport properties of SmFeAsO(0.7)F(0.25) single crystals reveals a promising combination of high (>2 x 10(6) A cm(-2)) and nearly isotropic critical current densities along all crystal directions. This favourable intragrain current transport in SmFeAs(O, F), which shows the highest T(c) of 54 K at ambient pressure, is a crucial requirement for possible applications. Essential in these experiments are four-probe measurements on focused-ion-beam-cut single crystals with a sub-square-micrometre cross-section, with current along and perpendicular to the crystallographic c axis.

High-pressure flux growth, structural, and superconducting properties ofLnFeAsO (Ln=Pr, Nd, Sm) single crystals

Single crystals of the LnFeAsO (Ln1111, Ln = Pr, Nd, and Sm) family with lateral dimensions up to 1 mm were grown from NaAs and KAs flux at high pressure. The crystals are of good structural quality and become superconducting when O is partially substituted by F (PrFeAsO1−xFx and NdFeAsO1−xFx) or when Fe is substituted by Co (SmFe1−xCoxAsO). From magnetization measurements, we estimate the temperature dependence and anisotropy of the upper critical field and the critical current density of underdoped PrFeAsO0.7F0.3 crystal with Tc ≈ 25 K. Single crystals of SmFe1−xCoxAsO with maximal Tc up to 16.3 K for x ≈ 0.08 were grown. From transport and magnetic measurements, we estimate the critical fields and their anisotropy and find these superconducting properties to be quite comparable to the ones in SmFeAsO1−xFx with a much higher Tc ≈ 50 K. The magnetically measured critical current densities are as high as 109 A/m2 at 2 K up to 7 T, with indication of the usual fishtail effect. The upper critical field estimated from resistivity measurements is anisotropic with slopes of ∼−8.7 T/K (H||ab plane) and ∼−1.7 T/K (H||c axis). This anisotropy (∼5) is similar to that in other Ln1111 crystals with various higher Tcs.

Strong magnetic pair breaking in Mn-substituted MgB2 single crystals

Magnetic ions (Mn) were substituted in MgB2 single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, Tc, in Mg1−xMnxB2 has been found to be rapidly suppressed at an initial rate of 10 K∕%Mn, leading to a complete suppression of superconductivity at about 2% Mn substitution. This reflects the strong coupling between the conduction electrons and the 3d local moments, predominantly of magnetic character, since the nonmagnetic ion substitutions, e.g., with Al or C, suppress Tc much less effectively (e.g., 0.5 K∕%Al). The magnitude of the magnetic moment (≃1.7 μB per Mn), derived from normal state susceptibility measurements, uniquely identifies the Mn ions to be divalent, and to be in the low-spin state (S=1∕2). This has been found also in x-ray absorption spectroscopy measurements. Isovalent Mn2+ substitution for Mg2+ mainly affects superconductivity through spin-flip scattering reducing Tc rapidly and lowering the upper critical field anisotropy Hc2ab∕Hc2c at T=0 from 6 to 3.3 (x=0.88% Mn), while leaving the initial slope dHc2∕dT near Tc unchanged for both field orientations.

Th-substituted SmFeAsO: Structural details and superconductivity with Tc above 50 K

We report structural, magnetic, and transport properties of polycrystalline samples and single crystals of superconducting Sm1−xThxFeAsO with maximal Tc above 50 K, prepared under high pressure. Bulk superconducting samples do not undergo a structural phase transition from tetragonal to orthorhombic symmetry at low temperatures. The unit-cell parameters a and c shrink with Th substitution and the fractional atomic coordinate of the As site zAs remains almost unchanged while that of Sm/Th zSm/Th increases. Upon warming from 5 to 295 K the increase in the FeAs layer thickness is dominant, while the changes in the other structural building blocks are minor, and they compensate each other, since the As-Sm/Th distance contracts by about the same amount as the O-Sm/Th expands. The polycrystalline and single-crystalline samples are characterized by a full diamagnetic response in low magnetic field, by a high intergrain critical current density for polycrystalline samples, and by a critical current density on the order of 8×105u2002A/cm2 for single crystals at 2 K in fields up to 7 T. The magnetic penetration depth anisotropy γλ increases with decreasing temperature, in a similar way to that of SmFeAsO1−xFy single crystals. The upper critical field estimated from resistance measurements is anisotropic with slopes of ∼5.4u2002T/K (H∥ab plane) and ∼2.7u2002T/K (H∥c axis), at temperatures sufficiently far below Tc. The low-temperature upper critical field anisotropy γH is in the range of ∼2, consistent with the tendency of a decreasing γH with decreasing temperature, previously reported for SmFeAsO1−xFy single crystals. n n© 2010 The American Physical Society

Influence of Mg deficiency on crystal structure and superconducting properties in MgB2 single crystals

The effects of high-temperature vacuum-annealing induced Mg deficiency in MgB2 single crystals grown under high pressure were investigated. As the annealing temperature was increased from 800 to 975 C, the average Mg content in the MgB2 crystals systematically decreased, while Tc remains essentially unchanged and the superconducting transition slightly broadens from 0.55 K to 1.3 K. The reduction of the superconducting volume fraction was noticeable already after annealing at 875 C. Samples annealed at 975 C are partially decomposed and the Mg site occupancy is decreased to 0.92 from 0.98 in as-grown crystals. Annealing at 1000 C completely destroys superconductivity. X-ray diffraction analysis revealed that the main final product of decomposition is polycrystalline MgB4 and thus the decomposition reaction of MgB2 can be described as 2MgB2(s) = MgB4(s) + Mg(g). First-principles calculations of the Mg1-x(VMg)xB2 electronic structure, within the supercell approach, show a small downshift of the Fermi level. Holes induced by the vacancies go to both sigma and pi bands. These small modifications are not expected to influence Tc, in agreement with observations. The significant reduction of the superconducting volume fraction without noticeable Tc reduction indicates the coexistence, within the same crystal, of superconductive and non-superconductive electronic phases, associated with regions rich and poor in Mg vacancies.

MgB2 single crystals substituted with Li and with Li-C: Structural and superconducting properties

The effect of Li substitution for Mg and of Li-C co-substitution on the superconducting properties and crystal structure of MgB2 single crystals has been investigated. It has been found that hole doping with Li decreases the superconducting transition temperature Tc, but at a slower rate than electron doping with C or Al. Tc of MgB2 crystals with simultaneously substituted Li for Mg and C for B decreases more than in the case where C is substituted alone. This means that holes introduced by Li cannot counterbalance the effect of decrease of Tc caused by introduction of electrons coming from C. The possible reason of it can be that holes coming from Li occupy the pi band while electrons coming from C fill the sigma band. The temperature dependences of the upper critical field Hc2 for Al and Li substituted crystals with the same Tc show a similar dHc2/dT slope at Tc and a similar Hc2(T) behavior, despite of much different substitution level. This indicates that the mechanism controlling Hc2 and Tc is similar in both hole and electron doped crystals. Electrical transport measurements show an increase of resistivity both in Li substituted crystals and in Li and C co-substituted crystals. This indicates enhanced scattering due to defects introduced by substitutions including distortion of the lattice. The observed behavior can be explained as a result of two effects, influencing both Tc and Hc2. The first one is doping related to the changes in the carrier concentration, which may lead to the decrease or to the increase of Tc. The second one is related to the introduction of new scattering centers leading to the modification of the interband and/or intraband scattering and therefore, to changes in the superconducting gaps and to the reduction of Tc.

Comment on “Superconducting anisotropy and evidence for intrinsic pinning in single crystallineMgB2”

In a recent paper [Phys. Rev. B 66, 012501 (2002)], torque data measured on a MgB2 single crystal in fields from 10 to 60 kOe at 10 K were presented. The authors obtained the anisotropy gamma by fitting a theoretical expression to the data and concluded that the anisotropy is field independent gamma approx 4.3. They also reported the observation of intrinsic pinning, which they take as experimental evidence for the occurence of superconductivity in the boron layers. In this comment, we discuss the range of validity of the theoretical expression used by the authors and show that the conclusion of a field independent anisotropy does not hold. Furthermore, we present torque data measured on two crystals of MgB2, establishing the extrinsic nature of the peak in the irreversible torque observed in some crystals.